Monthly Archives: July 2017

Even though they might look like it, those crystals in the picture above didn’t come out of a dog’s mouth. They are crystals of dogtooth calcite. Calcite (CaCO3) is the primary mineral that makes up limestone. It occurs in several crystal shapes. The two most commonly found in Arkansas are 6 sided rhombohedrons and the scalenohedral shape you see above. When it forms in this scalenohedral crystal structure it is called “dogtooth spar”.

Calcite is a very common mineral, but this particular crystal form of the mineral is typically only found in Arkansas in conjunction with the minerals sphalerite (zinc ore) and galena (lead ore) in the lead and zinc districts. Calcite is also a polymorph, like the mineral brookite from a previous geo-pic. This means calcite has “sister” minerals with the same chemical composition, but differing crystal structures. The three polymorphs of CaCO3 are: calcite, aragonite, and vaterite.

Geologic mapping of the Durham 7.5-minute quadrangle in northwest Arkansas was recently completed by the STATEMAP field team. STATEMAP in Arkansas is currently focused on detailed 1:24,000-scale mapping in the Ozark Plateaus Region in north Arkansas. It is accomplished through a cooperative matching-funds grant program administered by the US Geological Survey. Field work was performed between July and February, and included hiking/wading/swimming the entire 12-mile stretch of the upper White River located on the quad. Previous mapping delineated five stratigraphic units for the 1:500,000-scale Geologic Map of Arkansas, but at the 1:24,000 scale, we were able to draw ten. Further division is possible, but several units were considered too thin to map on the available 40-foot contour interval.

The Drakes Creek Fault, which runs diagonally from the southwest corner to the northeast corner, is the most striking feature on the map. It is part of a major structural feature in northwest Arkansas, forming a lineament that can be traced at the surface for over 45 miles. The Drakes Creek displays normal movement, is downthrown to the southeast, and offsets strata an average of 230 feet. Associated with the fault on the northwest side is a large drag fold. There, rocks parallel to the fault are deformed such that units typically present at higher elevations away from the fault bend down to a much lower elevation next to the fault. Erosion along this side of the fault has exposed the core of the fold along Fritts Creek, Cannon Creek, and other places.

Detail of Cross-section of Durham Quadrangle

The Durham quad is far-removed from areas of previous STATEMAP projects in north Arkansas. We completed work on the Mountain View 1:100,000-scale quad last year, ending on the Brownsville quad near Heber Springs. Focus has now turned to the Fly Gap Mountain 1:100K quad as the next high-priority area. When completed, we will have continuous 1:24K coverage for a large portion of the central Ozark Plateaus Region. The Durham quad was an appropriate choice to begin mapping in this area due to its proximity to designated type sections for many of the formations in north Arkansas. This facilitated easy comparisons between our field observations on Durham with the classic outcrops where these formations were first described. Initial field investigations included locating, describing, and sampling these historic outcrops near Fayetteville. We visited many places the names from which the stratigraphic nomenclature we still employ was derived. These places have such names as: Bloyd Mountain, Kessler Mountain, Lake Wedington, Cane Hill, Prairie Grove, Brentwood, Winslow, and Woolsey. Having seen the stratigraphy in these areas firsthand better prepares us to track changes in lithology and sedimentation as we continue to map to the east and south of Durham in the coming years.

The following images were taken during this year’s field season and are arranged in stratigraphic order from youngest to oldest: